Down syndrome is by far the most common and best known chromosomal disorder in humans and the most common cause of intellectual disability. It is characterized by intellectual disability , dysmorphic facial features, and other distinctive phenotypic traits.

What is Down syndrome? Down syndrome is a developmental disorder caused by an extra copy of chromosome 21 (which is why the disorder is also called "trisomy 21"). Having an extra copy of this chromosome means that each gene may be producing more protein product than normal. Cells seem to tolerate this better than having…

I'd like to tell you a curious story. Jane was a 52 year old woman in need of a kidney transplant. Thankfully she had three loving sons who were all very happy to give her one of theirs. So Jane's doctors performed tests to find out which of the three boys would be the best match, but the results surprised everyone. In the words of Jeremy Kyle, the DNA test showed that Jane was not the mother of two of the boys... Hang on, said Jane, child birth is not something you easily forget. They're definitely mine. And she was right. It turns out Jane was a chimera.
Chimerism is the existence of two genetically different cell lines in one organism. This can arise for a number of reasons- it can be iatrogenic, like when someone has an organ transplant, or it can be naturally occurring. In Jane's case, it began in her mum's womb, with two eggs that had been fertilised by different sperm creating two embryos. Ordinarily, they would develop into two non-identical twins. However in Jane's case the two balls of cells fused early in development creating one person with both cell lines.
Thus when doctors did the first tissue typing tests on Jane, just by chance they had only sampled the 'yellow' cell line which was responsible for one of her sons. When they went back again they found the 'pink' cell line which had given rise to the other two boys.
This particular type of human chimerism is thought to be pretty rare- there are only 30 case reports in the literature. (Though remarkably both House and CSI's Gil Grissom have encountered cases.) What happens far more frequently is fetal microchimerism- which occurs in pregnant women when cells cross the placenta from baby to mum. This is awesome because we used to think the placenta was this barrier which prevented any cells crossing over. Now we've found both cells and free floating DNA cross the placenta, and that the cells can hang around for decades after the baby was born. Why? As is often the case in medicine we're not sure but one theory is that the fetal cells might have healing properties for mum. In pregnant mice who've had a heart attack, fetal cells can travel to the mum's heart where the develop into new heart muscle to repair the damage.
Whilst we're still in the early stages of understanding why this happens, we already have a practical application. In the United States today, a pregnant woman can have a blood test which isn't looking for abnormalities in her DNA but in that of her fetus. The DNA test isn't conclusive enough to be used to diagnose genetic conditions, but it is a good screening test for certain trisomies including Down's syndrome.
Now, we started with a curious tale, so lets close with a curious fact, and one that's appropriate for Mother's Day: This exchange of cells across the placenta is a two way process. So you may well have some of your mum's cells rushing through your veins right now. In my case they're probably the ones that tell me to put on sensible shoes and put that boy down...
(FYI: This is a story I originally posted on my own blog)

Although cancer has an annual incidence of only about 150 new cases per 1 million U.S. children, it is the second leading cause of childhood deaths. Early detection and prompt therapy have the potential to reduce mortality. Leukemias, lymphomas and central nervous system tumors account for more than one half of new cancer cases in children. Early in the disease, leukemia may cause nonspecific symptoms similar to those of a viral infection. Leukemia should be suspected if persistent vague symptoms are accompanied by evidence of abnormal bleeding, bone pain, lymphadenopathy or hepatosplenomegaly. The presenting symptoms of a brain tumor may include elevated intracranial pressure, nerve abnormalities and seizures. A spinal tumor often presents with signs and symptoms of spinal cord compression. In children, lymphoma may present as one or more painless masses, often in the neck, accompanied by signs and symptoms resulting from local compression, as well as signs and symptoms of systemic disturbances, such as fever and weight loss. A neuroblastoma may arise from sympathetic nervous tissue anywhere in the body, but this tumor most often develops in the abdomen. The presentation depends on the local effects of the solid tumor and any metastases. An abdominal mass in a child may also be due to Wilms' tumor. This neoplasm may present with renal signs and symptoms, such as hypertension, hematuria and abdominal pain. A tumor of the musculoskeletal system is often first detected when trauma appears to cause pain and dysfunction out of proportion to the injury. Primary care physicians should be alert for possible presenting signs and symptoms of childhood malignancy, particularly in patients with Down syndrome or other congenital and familial conditions associated with an increased risk of cancer.

People with Down syndrome often experience cognitive decline (a deterioration in memory, language, thinking and judgment that are greater than normal age-related changes) at a younger age and in greater numbers than the general population. Various medicines have been shown to improve, or at least slow down the progression of these symptoms in people without Down syndrome.